As international interests have recently risen in the global environment, securing future clean energy sources has emerged as an important subject at a national security level as well as in economic aspects. Accordingly, all countries of the world have made much invest on the development of new and renewable energy for replacing fossil fuel. However, current new and renewable energy has many disadvantages compared to traditional energy sources in that economic feasibility is not secured, supply contribution is low, high maintenance costs are required and the like.
In addition, as electronic goods, electronic devices, communication devices and the like have rapidly become smaller and lighter, and necessity of electric vehicles has highly emerged regarding environmental problems, demands for improving performance of secondary batteries used as a power source of these goods have greatly increased. Among these, lithium batteries have received considerable attention as a high performance battery due to their high energy density and high standard electrode potential.
Particularly, lithium-sulfur (Li—S) batteries are a secondary battery using a sulfur series material having sulfur-sulfur (S—S) bonds as a positive electrode active material, and using lithium metal as an anode active material. Sulfur, a main material of a positive electrode active material, has advantages of being very abundant in resources, having no toxicity and having a low atomic weight. Moreover, a lithium-sulfur battery is a most promising battery among batteries that have been developed so far in terms of energy density with lithium metal used as an anode active material having theoretical capacity of 3860 mAh/g and sulfur (S8) used as a positive electrode active material having theoretical capacity of 1675 mAh/g.
During a discharge reaction of a lithium-sulfur (Li—S) battery, an oxidation reaction of lithium occurs in an anode, and a reduction reaction of sulfur occurs in a positive electrode. Sulfur has a cyclic S8 structure before discharge, and electric energy is stored and produced using an oxidation-reduction reaction in which an oxidation number of S decreases as S—S bonds are broken during a reduction reaction (during discharge), and an oxidation number of S increases as S—S bonds are formed again during an oxidation reaction (during charge). During such a reaction, the sulfur is converted to linear-structured lithium polysulfide (Li2S2, Li2S4, Li2S6, Li2S8) from cyclic S8 by the reduction reaction, and lithium sulfide (Li2S) is finally produced when such lithium polysulfide is completely reduced. By the process of being reduced to each lithium polysulfide, discharge behavior of a lithium-sulfur (Li—S) battery shows discharging voltages gradually unlike lithium ion batteries.
Accordingly, when assuming that sulfur (S8) completely reacts with lithium to lithium polysulfide (Li2S), a discharge product, the theoretical discharging capacity is 1,675 mAh/g-sulfur and the theoretical energy density is 2,600 Wh/kg, which are 3 times to 6 times higher than theoretical discharging capacity of current lithium ion batteries (570 Wh/kg) and currently studied other battery systems (Ni-MH: 450 Wh/kg, Li—FeS: 480 Wh/kg, Li—MnO2: 1,000 Wh/kg, Na—S: 800 Wh/kg). High energy density is caused by high specific capacity of sulfur and lithium. However, high utilization of sulfur and lithium needs to be secured for accomplishing such values.
So far, there have been no examples of successful commercialization as a lithium-sulfur (Li—S) battery system. The reason for unsuccessful commercialization of lithium-sulfur (Li—S) battery is that, first of all, when using sulfur as an active material, utilization representing the amount of sulfur participating in an electrochemical oxidation and reduction reaction in a battery is low with respect to the amount of the introduced sulfur, and, unlike theoretical capacity, extremely low battery capacity is obtained actually.
In addition, elemental sulfur is generally a nonconductor having no electrical conductivity, and therefore, an electrical conductor capable of providing a smooth electrochemical reaction site needs to be used for an electrochemical reaction to occur.
As a conductor material currently used, carbon-based such as Ketjen black, carbon black, Super-P, carbon nanofiber (CNF) and multi-walled carbon nanotubes (MWNT) has been used. The carbon-based has a large specific surface area, which is very advantageous in terms of increasing a contact surface between sulfur and an electrolyte. However, carbon black is amorphous carbon, and therefor has poor lithium ion intercalation characteristics, and sometimes becomes a cause for the occurrences of irreversible capability. Carbon nanotubes are high crystalline carbon and thereby has excellent electrical conductivity, and may perform a role of a path through which sulfur in an electrode react with lithium ions. In addition, carbon nanotubes have a linear mesh structure, and therefore, have structural stability in a sulfur electrode.
A conductor electrically connects an electrolyte and sulfur, and performs a role of a path enabling lithium ions (Li+) dissolved in the electrolyte to migrate to and react with the sulfur. At the same time, the conductor also performs a role of a path for electrons to migrate from a current collector to the sulfur. Accordingly, when the amount of the conductor is not sufficient or the conductor does not properly perform its roles, unreacted portions increase among the sulfur in the electrode resultantly causing a capacity decrease. In addition, this also adversely affects high rate discharge characteristics and charge and discharge cycle life. Accordingly, a proper conductor needs to be added.
As described in U.S. Pat. Nos. 5,523,179 and 5,582,623, positive electrode structures using elemental sulfur known so far have a structure in which sulfur and carbon powder, a conductor, are each independently present and simply mixed in a positive electrode active material layer (mixture). However, in such a structure, the electrode structure is collapsed when sulfur is eluted to an electrolyte in a liquid form as the sulfur becomes polysulfide when charged and discharged, which adversely affects capacity and lifespan characteristics of a lithium-sulfur (Li—S) battery.